Northern Ireland's Chief Constable could be summoned before a coroner's court over claims that police are dragging their heels on controversial "shoot-to-kill" inquests.

Matt Baggott may be asked to explain the six-year delay in releasing sensitive documents relating to the deaths of six men killed by the security forces during the 1980s.

At a preliminary hearing in Belfast there were also calls for long-classified investigations by Greater Manchester Police Deputy Chief Constable John Stalker and Sir Colin Sampson, of West Yorkshire Police, to be made public.

Barrister Barry Macdonald QC, acting for the families of some of those killed, said the situation was unacceptable.

" It is only explicable on the basis that the chief constable is seeking to stall this process and prevent disclosure of sensitive material to the families for as long as can be achieved," he said.

"That has been the pattern since these incidents occurred and since this inquest process was started. It is completely unacceptable."

Mr Macdonald added: "Perhaps it is time to require the chief constable to appear to explain in person."

The case involves six people, including IRA men and a Catholic teenager, who were shot dead by the security forces around Lurgan and Armagh in 1982 amid claims there was a deliberate intention to kill them.

The coroner will also examine the deaths of three Royal Ulster Constabulary (RUC) officers who died in a bomb blast weeks earlier, an attack allegedly carried out by the IRA men who were subsequently gunned down and therefore seen as a potential motivation for the claimed shoot-to-kill policy.

During a brief hearing in Belfast's Old Town Hall it was revealed that 697 pages of documents had recently been disclosed to relatives' lawyers.

But Tony McGleenan QC, representing the Police Service of Northern Ireland (PSNI), was unable to give a time-frame for full disclosure of up to 17 batches of material.

He described as unprecedented the process of reading and redacting to remove information that could potentially threaten national security and outlined resourcing issues for the PSNI.

Some of the highly sensitive material can only be viewed by a small number of individuals including the Chief Constable or Justice Minister David Ford, it was also claimed.

Mr McGleenan said: "I am not able to put an end point. We are working to finish tranche four. The next stage is tranche five. I would not expect that the process would be complete before the summer of 2014."

Acting for the coroner's office, Frank O'Donohue QC said he shared frustrations about the slowness of the inquest process and would reflect on the suggestion of bringing Mr Baggott to court.

The Stalker and Sampson reports were long classified top secret but the PSNI finally handed over edited versions to the coroner in 2010 after a long legal battle. The reports were then passed to lawyers for the families.

Karen Quinlivin QC, who is also representing some of the relatives, said the PSNI had taken an obstructionist approach to inquest proceedings and were drip-feeding material.

She called for the Stalker/Sampson reports, which are currently only available to legal representatives who sign a confidentiality agreement, to be made public.

"This is material that is being withheld from the public," said Ms Quinlivin. "There is no good reason why the Stalker/Sampson report, as we have seen it in redacted form, should not be available to the media.

"That material is damning of the officers involved and ought to be aired in the public forum."

Paul Doran QC, who is representing the families of the three police officers killed in the IRA bomb attack, also aired concerns at the six-year time lapse.

"As lawyers we obviously get frustrated but our concerns pale into insignificance alongside the anxiety and distress of the next of kin. Hopes are raised when inquests such as these are put on to an agenda," he said.

Describing the delay as concerning, senior coroner John Leckey said there was a duty on the state to provide adequate resources for an inquest to be held within a reasonable period of time.

He said: "These inquests constitute the state's mechanism in dealing with death. If the chief constable is able to proceed at such a slow pace, is there a duty on the state to do something to accelerate this process? If the answer to that is yes, then what duty lies with the Minister (of Justice)?"

Mr Leckey said he would raise the issues in a letter to the Justice Minister.

It was also revealed that the elderly parents of some of the victims had died without hearing the full details of what happened to their sons.

Mr Leckey added: "We are aware there is an ageing generation and some of that generation is passing on. I find it very sad that is happening without them being about to hear what is happening in relation to the inquest into these deaths."

Speaking outside the court, Mark Thompson, from victims' group Relatives for Justice, said families were angry at the constant delay.

"We would call on the Chief Constable and Minister for Justice to explain the delays. This is unacceptable and just cannot go on. The state needs to tell the truth," he said.

In a statement issued after the preliminary hearing, lawyers for the families of Gervaise McKerr, Eugene Toman and Sean Burns said: "The delays in this case can be attributed solely to the chief constable.

"In view of the continued delay in the provision of disclosure and the consequent delay in public hearings, the families are calling on the chief constable to release the Stalker/Sampson reports more generally so that the wider public are informed about the seriousness of the allegations made against RUC officers, some of whom it is understood are still serving."

Solicitor Niall Murphy added: "Our clients are entitled to a transparent examination of the murders of the loved ones, indeed the State is legally obliged to convene one, however it appears that the chief constable and the security services are implacably opposed to that."

Meanwhile, Sinn Fein described the six-year delay over legacy inquests as an affront to democracy.

Upper Bann MLA John O'Dowd said: "Domestic courts, European courts and the coroner's court have ruled that the PSNI must cooperate with the inquest.

"Further delay in the PSNI handing over all papers is an affront to justice and democracy.

"The PSNI must not now stop delaying these inquests or be seen to fail both justice and democracy."